1. FIELD OF THE INVENTION
The present invention relates to a strobe device including an insulated gate bipolar transistor (hereinafter referred to as "IGBT") connected in series with a flash discharge tube for controlling the luminous operation of the flash discharge tube. In particular, the present invention relates to a strobe device having an improved voltage supply section for supplying a voltage to the flash discharge tube, thereby providing an advantage in the case where the flash discharge tube repeats luminous emissions at a high speed, for example.
2. DESCRIPTION OF THE RELATED ART
A variety of strobe devices including an IGBT connected in series with a flash discharge tube for controlling the luminous operation of the flash discharge tube have been proposed or implemented.
Well-known conventional strobe devices incorporating an SCR as an element for controlling the luminous operation require a terminating structure. In contrast, the above strobe device incorporating an IGBT does not require any terminating structure in the circuitry thereof. Therefore, such a strobe device prevents overflash problems and stably enables repetitive luminous operations at a high cycle. Consequently, this type of strobe device has been regarded as especially useful in recent years.
A strobe device is also known which includes a step-up means, i.e., a means for stepping up the voltage across a flash discharge tube by applying a predetermined voltage across the flash discharge tube in synchronization with an operation for starting the luminous emission (hereinafter referred to as a "start operation"), the predetermined voltage being higher than the charged voltage of a main capacitor. The step-up means is incorporated, in addition to a known trigger structure, as an auxiliary means for starting the luminous operation of the flash discharge tube. Such a step-up means is conveniently incorporated also in the aforementioned strobe device having an IGBT.
FIGS. 6A and 6B show exemplary strobe devices incorporating a step-up means as described above, which was previously proposed by the Applicant in U.S. Pat. No. 5,180,953. FIG. 6A shows an essential portion of the circuitry of an exemplary IGBT-type strobe device configured so that a voltage more than about twice as high as the charged voltage of a main capacitor 2 is constantly applied across e flash discharge tube 3 during a start operation of the flash discharge tube 3. Similarly, FIG. 6B shows an essential portion of the circuitry of an exemplary IGBT type strobe device configured so that a voltage about three times as high as the charged voltage of a main capacitor 2 is constantly applied across a flash discharge tube 3 during a start operation of the flash discharge tube 3.
In each of the strobe devices shown in FIGS. 6A and 6B, the main capacitor 2 is coupled between output terminals of a DC high voltage power supply 1 (which may be composed of a known DC-DC convertor or a multilayer power supply) for charging the main capacitor 2.
Across the main capacitor 2 in FIG. 6A, the flash discharge tube 3, a first diode 4, and an IGBT 5 are connected in series. A resistor 6, a transistor 7, and a second diode 8 are connected in series across serially connected elements including the first diode 4 and the IGBT 5. The transistor 7 includes a base as a control terminal for the transistor 7 functioning as a switch.
A multiplying capacitor 9 is connected between a junction X (between the first diode 4 and the IGBT 5) and a junction Y (between the transistor 7 and the second diode 8). Resistors 10 and 11 are connected in series across the second diode 8. A Junction between the resistors 10 and 11 is connected to the base (i.e., control terminal) of the transistor 7. Between a higher-potential terminal of the main capacitor 2 and the junction X, a resistor 12 may be connected, for example; the resistor 12 and the second diode 8 form a charge circuit for charging the multiplying capacitor 9 so as to provide a higher potential at the junction X. Across the IGBT 5, a known trigger circuit 15 for exciting the flash discharge tube 3 is connected, the trigger circuit 15 including a trigger capacitor 13 and a trigger transformer 14.
An emission control circuit 16 coupled to the gate of the IGBT 5 controls the supply of an ON voltage to the gate of the IGBT 5, thereby turning the IGBT 5 on or off. Thus, the emission control circuit 16 controls the luminous operation of the flash discharge tube 3.
Turning to the exemplary IGBT type strobe device shown in FIG. 6B, the flash discharge tube 3, a plurality of diodes 17 and 18, and the IGBT 5 are connected in series across the main capacitor 2.
A set of serially connected elements including transistor 19 and step-up capacitor 21, and a set of serially connected elements including transistor 20 and step-up capacitor 22 are connected between an anode and a cathode of each of the plurality of diodes 17 and 18, respectively. Furthermore, a diode 23 for preventing a reverse current is connected between an emitter of the IGBT 5 (shown to be grounded in the figure) and a junction between the transistor 19 and the step-up capacitor 21. Similarly, a diode 24 for preventing a reverse current is connected between the emitter of the IGBT 5 and a junction between the transistor 20 and the step-up capacitor 22.
Resistors 25 and 27 are connected between the emitter of the IGBT 5 and the base of the transistor 19; resistors 26 and 27 are connected between the emitter of the IGBT 5 and the base of the transistor 20. A resistor 28 is connected between the base and the emitter of the transistor 19; and a resistor 29 is connected between the base and the emitter of the transistor 20.
Across the flesh discharge tube 3, a resistor 12 may be connected, for example; the resistor 12 and the diode 23 or 24 form a charge circuit for charging the step-up capacitor 21 or 22 so that a higher potential is provided at its terminal coupled to the collector of the IGBT 5.
As in the exemplary IGBT type strobe device shown in FIG. 6A, a known trigger circuit 15 for exciting the flash discharge tube 3 is connected across the IGBT 5. An emission control circuit 16 is coupled to the gate of the IGBT 5 for controlling the supply of an ON voltage to the gate of the IGBT 5 so as to turn the IGBT 5 on or off, thereby controlling the luminous operation of the flash discharge tube 3.
Thus, in accordance with the device of either FIG. 6A or 6B, as the DC high voltage power supply 1 starts operating, the main capacitor 2, the multiplying capacitor 9, the trigger capacitor 13, and the step-up capacitors 21 and 22 are charged so as to have the polarities as shown in FIG. 6A or 6B, respectively.
When the emission control circuit 16 turns on the IGBT 5 after the main capacitor 2 is charged, the trigger circuit 15 operates to excite the flash discharge tube 3. At the same time, in the strobe device of FIG. 6A, the electricity charged in the multiplying capacitor 9 is discharged via the IGBT 5 and the resistors 10 and 11. As a result, the transistor 7 is turned on due to a drop in the voltage across the resistor 10.
In the strobe device of FIG. 6B, the electricity charged in the step-up capacitors 21 and 22 is discharged via the path including the IGBT 5 and the resistors 27, 25, and 28, and via the path including the IGBT 5 and the resistors 27, 26, and 29, respectively. As a result, the transistors 19 and 20 are turned on due to a drop in the voltage across the resistors 28 and 29, respectively.
As a result, in the strobe device of FIG. 6A, the charged voltage of the multiplying capacitor 9 is applied across the flash discharge tube 3 via a loop including the IGST 5, the main capacitor 2, and the transistor 7. Thus, a superimposed voltage of the main capacitor 2 and the multiplying capacitor 9, which is about twice as high as the charged voltage of main capacitor 2, is applied across the flash discharge tube 3.
In the strobe device of FIG. 6B, the charged voltages of the step-up capacitors 21 and 22 are applied across the flash discharge tube 3 via a loop including the IGBT 5, the main capacitor 2, and the transistors 19 and 20. Thus, a superimposed voltage of the main capacitor 2 and the multiplying capacitors 21 and 22, which is about three times as high as the charged voltage of the main capacitor 2, is applied across the flash discharge tube 3.
Therefore, in either case, the flash discharge tube 3 begins to consume the charged electricity of the main capacitor 2 as the IGBT 5 is turned on. Thus, the flash discharge tube 3 easily flashes.
If the IGBT 5 is turned off by the emission control circuit 16 at an appropriate point of time during the emission of the flash discharge tube 3, the discharge loop including the main capacitor 2 and the like via the IGBT 5 is disrupted, in accordance with the strobe device of either FIG. 6A or 6B. As the flash discharge tube 3 consequently stops luminous emission, the flash discharge tube 3 experiences an ionization state where its cathode potential becomes high, and eventually returns to a normal state, as is well known to those skilled in the art. At the same time, the multiplying capacitor 9 and the like are placed in a state for enabling charging.
Thus, in the strobe device of FIG. 6A, a current flows in a loop including the main capacitor 2, the flash discharge tube 3, the multiplying capacitor 9, and the second diode 8; and a loop including the main capacitor 2, the flash discharge tube 3, the trigger capacitor 13, and the trigger transformer 14.
In the strobe device of FIG. 6B, a current flows in a loop including the main capacitor 2, the flash discharge tube 3, the step-up capacitor 21, and the reverse current-prevention diode 23; a loop including the main capacitor 2, the flash discharge tube 3, the step-up capacitor 22, and the reverse current-prevention diode 24; and a loop including the main capacitor 2, the flash discharge tube 3, the trigger capacitor 13, and the trigger transformer 14.
As a result, the multiplying capacitor 9, the step-up capacitors 21 and 22 and the trigger transformer 13 are charged instantaneously.
Thus, it is ensured that the charged voltage across the multiplying capacitor 9 or the step-up capacitors 21 and 22 is applied again across the flash discharge tube 3 when the IGBT 5 is turned on for a next luminous operation.
As described above, the strobe device shown in FIG. 6A or 6B includes the multiplying capacitor 9 or the step-up capacitors 21 and 22 capable of rapid charging as the flash discharge tube 3 enters an ionization state. As a result, a predetermined function of increasing the voltage across the flash discharge tube 3 is performed each time by applying the charged voltage of the multiplying capacitor 9 or the step-up capacitors 21 and 22 across the flash discharge tube 3 when the IGBT 5 is turned on. Thus, the flash discharge tube 3 can flash in time each time the IGBT 5 is turned on, even in a high frequency cycle.
However, the above-mentioned advantage of the strobe devices shown in FIGS. 6A and 6B requires a number of elements incorporated corresponding to the multiplying capacitor 9, and the step-up capacitors 21 and 22.
In other words, a set of elements are required for each multiplying capacitor 9 or step-up capacitor 21, etc., each set including the transistors 7, 19 and 20; and the set of resistors 6, 10, 11, and the set of resistors 25 to 29 for controlling the operations of the transistors 7, 19 and 20. As a result, the total number of elements required for the strobe device of FIG. 6A or 6B becomes rather large, with correspondingly larger space and higher cost being required.
For example, the transistor 7 in FIG. 6A and the transistor 19 in FIG. 6B thus coupled are required to withstand a high voltage, and also require the resistors 10, 25, and the like for operation. Such are the burden on the space and cost of the above-described conventional strobe device.